The present invention relates to an impeller in a centrifugal blower, the front and/or back plate of the impeller being made up of a center part and a radially outer part attached to it and having a material thickness smaller than that of the center part. The front and back plates of the impeller are plate disks. In rotating plate disks, stresses increase towards the hub. The stresses are tensile. Obtaining a disk of even strength by reducing its thickness towards the periphery is previously known. Likewise, the manufacture of impellers by welding is part of current technology. Furthermore, prestressing of structures is used in both construction technology and mechanical engineering, as is also prestressing by means of welding. It is also known that different stresses prevail in different parts of the impeller of a blower, and owing to the aerodynamic form it has not been possible entirely to even out these stresses by varying the thickness of material. Prestressing of the structure is in this case the only usable method by which stress peaks can be lowered. Several attempts have been made. In one structure the front plate is tied to the hub by means of tensing bars, which are prestressed. The characteristics that attempts of this type have in common is that they require additional parts which result in additional costs, limit the blower's uses and lower the performance of the blower. In the case mentioned, the tensing bars increase the noise produced by the blower and lower its efficiency. The lowering of stress peaks achieved by these means is quite modest.